Self-stabilizing combustion apparatus



P 1966 J. D. NESBITT 3,273,622

SELF-STABILIZING COMBUSTION APPARATUS Original Filed April 11, 1961 4 Sheets-Sheet 1 (To FURNACE TEMPERATURE DETECTOR) l l I FURNACE 2 TE MPERATURE CONTROLLER JOHN D. NESBlTT AT TORNY Sept. 20, 1966 J. D. NESBITT 3,

SELF-STABILIZING COMBUSTION APPARATUS Original Filed April 11, 1961 4 Sheets-Sheet 2 INVENTOR. JOHN D. NESBITT ATTORNEY Sept. 20, 1966 J. D. NESBITT SELFSTABILIZING COMBUSTION APPARATUS Original Filed April 11, 1961 4 Sheets-Sheet 5 INVENTOR. JOHN D. NESBITT ATTORNEY United States Patent 2 Claims. e1. 1ss-7 This is a division of application Serial No. 473,267, filed June 7, 1965, now Patent No. 3,244,219 issued April 5, 1966, which in turn is a division of application Serial No. 102,316, filed April 11, 1961, now Patent No. 3,187,799 issued June 8, 1965, which in turn is a continuation in part of application Serial No. 808,161 filed April 22, 1959, now abandoned.

This invention relates to a system for stabilizing combustible reaction of streams of fuel and air in various types of combustion apparatus. More particularly, the invention relates to apparatus for making various types of combustion equipment self-stabilizing.

The stabilization of the combustible reactions promoted by various types of industrial combustion apparatus is frequently accomplished by means of a separate combustion system, normally of the pre-mix type, used as a piloting system. The use of a separate piloting system to provide flame stability for a principal combustion system has a number of undesirable features among which are the complication of the structure of the burners of the principal combustion system to make them suitable to receive a pilot burner, complication of the piping required to pipe the overall system, and, in the case of a premix piloting system, certain well recognized safety hazards.

It is, therefore, an object of my invention to overcome the dependency of prior art combustion system on separate piloting systems for proper flame stability. It is a further object of this invention to accomplish the aforesaid object by providing self-stabilizing combustion apparatus.

For further consideration of what I believe to be novel and may invention, attention is directed to the preferred form of my invention as disclosed in the following portion of the specification, the appended claims, and the accompanying drawing.

FIG. 1 illustrates an elevational view, in section, of a burner embodying the present invention.

FIG. 2 illustrates a view taken on line 2-2 of FIG. 1.

FIG. 3 illustrates an elevational View, in section of a burner embodying the present invention, the burner being specifically adapted to fire into an immersion heating tube.

. FIG. 4 illustrates an elevational view of a radiant heating tube of the hairpin type having a burner embodying the present invention.

FIG. 5 illustrates an elevational view, in section, of an alternative burner construction embodying the present invention.

FIG. 6 illustrates a view taken on line 66 of FIG. 5.

FIG. 7 illustrates an elevational view, partially in section, of a single pass radiant tube having a burner of alternative construction embodying the present invention.

FIG. 8 illustrates a view of the burner of FIG. 7 taken on line 88.

FIG. 9 illustrates a view taken on line 9-9 of FIG. 8.

Referring in more detail to the drawing and in particular to FIGS. 1 and 2 a burner 10 is shown thereat installed in a furnace wall. A typical application for this burner is in a low temperature tempering furnace. Burnor 10 comprises a tunnel block 11 attached to burner body 12. Tunnel block 11 is of refractory composition 3,273,622 Patented Sept. 20, 1966 and square configuration with a cylindrical tunnel chamber 13 extending longitudinally therethrough. The burner body 12 which is generally made of cast steel, is adapted to be installed in a refractory lined furnace wall 14. Burner body 12 is attached to metallic casing 15 of furnace wall 14 by means of studs 16 affixed to casing 15 and extending through holes 17 in burner body 12. Nuts 18 threaded on studs 16 securely hold the burner body to the casing, and the studs are so positioned that the body is properly aligned with respect to cylindrical tunnel chamber 13.

Burner body 12 has backplate 19 attached by means of studs 21 afiixed to *body 12 and extending through holes 22 in plate 19. Nuts 23 threaded on studs 21 securely hold the backplate to the burner body.

Disposed within the burner body 12 is wall means forming a substantially cylindrical combustion chamber 24 axially aligned with tunnel chamber 13 and welded to backplate 19. Combustion chamber 24 has an outwardly expanding or tapered section 25 at its exit end for approximately /3 of the total chamber length. The tapered section expands from the cylindrical portion 26 of chamber to approximately the internal diameter of tunnel chamber 13. The tapered section is used merely to provide smooth expansion of the gases from cylindrical portion 26 to the tunnel chamber. In a larger capacity burner, the same burner body 12 can be used with a larger cylindrical combustion chamber equal in diameter to the tunnel chamber diameter and having no tapered portion. For example, the combustion chamber 124 of the burner in FIG. 3 is shown as a straight cylindrical casting.

A fuel pipe 27 is welded to backplate 19 flush with the inside face thereof and in substantially axial alignment with combustion chamber 24 and tunnel chamber 13. Pipe 27 is supplied fuel from a source, not shown, through lines 28, 29 and 31 the latter two lines containing valves 32 and 33 respectively. Backplate 19 is also provided with two tapped holes 34 and 35 into which pipe plugs 36 and 37 are inserted. Holes 34 and 35 are located equidistant from the axis of and within the combustion chamber when the latter is welded to the backplate. The burner can be ignited through one of the holes 34 and 35 by an electric or fuel torch. As will be explained later, the combustion chamber will normally be under positive pressure, and unless the burner is ignited at throttled air flow, it may be difficult to ignite because of outflow through the hole. For gaseous fuel operation, the preferred form of ignition will normally be an electric spark inserted into one of the holes 34 and 35 in place of pipe plug 36 or 37.

A commercially available flame rod safety device may be placed in the other of the holes 34 and 35 in place of plug 36 or 37 shut off a valve in line 28 should the flame in combustion chamber 24 become extinguished.

Air inlet pipes 38 and 39, FIG. 2, are welded tangent to the internal diameter of combustion chamber 24 adjacent backplate 19. Pipes 38 and 39 are directed perpendicularly to the axis of chamber 24 and are normally spaced from the backplate 19 a short distance such as to /2" and, as shown in FIG.'2, they are spaced 180 apart and attached to the combustion chamber in a manner to introduce the air in the same direction, in this case counterclockwise. Inlet pipes 38 and 39 are shown entering combustion chamber 24 adjacent holes 34 and 35 in backplate 19. They are shown in this manner for convenience and could be located spaced from their present location, or under some circumstances it may be desirable to supply four inlet pipes 90 apart. Inlet pipes 38 and 39 must be long enough in relation to their internal diameter to provide a straight air flow path. A length to inside diameter ratio of at least 2 to 1 has proven satisfactory. By virtue of the perpendicular 3 disposition of inlet pipes 38 and 39 with respect to the axis of chamber 24 the air admitted therethrough will have substantially no component of motion parallel to the axis of chamber 24.

Additional air is admitted to combustion chamber 24 by a plurality of apertures 41 in the chamber wall means downstream of the tangential air inlet pipes 38 and 39. The size and location of apertures 41 are such as to maintain combustion of the fuel stream in combustion chamber 24 at a maximum rate without flame quenching. A convenient pattern for spacing apertures 41 to obtain radial entry of air is in a plurality of axially spaced circumferential stages. The burner of FIG. 1 shows four of these stages downstream of the tangential air inlet pipes 38 and 39. The first three stages are in cylindrical portion 26 of the combustion chamber, and each stage contains four apertures circumferentially spaced 90 apart. The fourth stage is located in outwardly tapered section 25, and it contains eight apertures circumferentially spaced 45 apart.

Air entering each aperture 41 will produce a back eddy current which will promote stable burning with maximum mixing. As previously stated, the size and spacing of apertures 41 is such as to maintain combustion at the maximum possible rate without flame quenching. Flame quenching is indicated by the presence of intermediate combustion products. The presence of intermediate combustion products can be qualitatively detected, even in extremely minute quantities, by the characteristic pungent odor of the aldehydes formed. Once flame quenching is known to exist by the detection of aldehyde odors, the extent of the flame quenching can be evaluated quantitatively by measurement of carbon monoxide content in the combustion gases. In this manner, the optimum aperture arrangement can be accurately determined for burners of different capacity.

The space between burner body 12 and combustion chamber 24 defines an annular air chamber 42. Air is supplied to this chamber by a blower and piping, not shown, to flange portion 43 of burner body 13. Air is then properly distributed between tangential air inlet pipes 38 and 39 and radial apertures 41 by their relative areas.

In excess air operation of the burner, a small percentage of the total air supplied to air chamber 42 is introduced to combustion chamber 24 through tangential air inlet pipes 38 and 39 to form a spinning air stream circumposing an axially flowing fuel stream emitted to the chamber through fuel pipe 27. The combustible mixture of fuel and air formed at the interface of the spinning air stream and the axially flowing fuel stream is ignited through one of the apertures 34 and 35 in backplate 19 to establish a spinning, slow mixing, stable flame adjacent the gas nozzle. This spinning flame, in effect, establishes an extremely stable pilot for the balance of the fuel and arr.

The preferred amount of air emitted through the tangential air inlet pipes is approximately 57.5%, with 4- of the total air being acceptable. If too little of the total air is admitted to the tangential air inlet pipes, for example, less than 4%, the air will pull out in a parabola along the gas stream to produce an unstable flame which will finally quench as the spin air is further decreased. When too much of the total air is admitted tangentially, over approximately 10%, a back eddy or vortex is created which causes carbon to build up on the backplate which results in the flame rod and ignition means inserted through holes 34 and 35 in the backplate becoming inoperative, and the general maintenance of the burner is greatly increased.

The preferred air pressure drop is 1.5 to 3.5" W.C. with operation at approximately 2" W.C. being standard. Increasing the pressure beyond 3.5" W.C. results in flame quenching and the consequent production of aldehydes.

Fuel is supplied to fuel pipe 27 at 0 to 1" W.C. pressure. Under normal excess air operation the air flow is maintained constant, and the fuel is cut back as heat demand decreases. Because of the extremely wide range of excess air that may be used, any of the common control systems may be used. With the valve arrangement of FIG. 1, a simple high-low control system can be used. When the burner is firing at full capacity, both valves 32 and 33 will be open to supply fuel from line 28 through lines 29 and 31. When heat demand decreases, a simple temperature controller, not shown, can be used to close valve 33, and only a small percentage of maximum fuel flow, for example, 3 to 5% will be supplied from line 28 through line 29 and open valve 32. A proportional or modulating control system can also be used wherein one of the valves 32 and 33 would be a motor driven valve throttled by a temperature controller an amount which is proportional to the deviation between set point or desired furnace temperature and actual furnace temperature.

Excess air burners of this type can be commonly built with capacities ranging from 300,000 to 3,000,000 B.t.u./ hr. with a corresponding turndown ratio (maximum to minimum fuel flow) of from 30/1 to /1. The prior patent art indicates that the heating capacity of luminous or long flame burners ranges up to approximately a maximum of 70,000 B.t.u. per hour per cubic foot of combustion space; whereas, the burner of the present invention has a heating capacity of 1,000,000 B.t.u. per hour per cubic foot of combustion space.

Burner 110, shown in FIG. 3, is a modification of burner 10 and is particularly designed for heating immersion tubes. The similarity of elements of the embodiments of FIGS. 3 through 9 to the embodiment of FIGS. 1 and 2 is indicated by identifying them with a three digit numeral whose last two digits-are the same as the two digits of the numeral used to identify the corresponding element of the embodiment of FIGS. 1 and 2.

In heating sealed immersion tubes, it is desirable to compromise between the rapid mixing, high capacity burner 10 of FIG. 1 and the prior art long flame burner. If burner 10 were used without modification for this application, a hot spot would result adjacent the point where the burner is joined to the tube because combustion is rapid and is completed close to the exit of the combustion chamber even when firing at maximum capacity. It is desirable to complete combustion uniformly along the length of the tube so that the tube becomes uniformly heated.

Burner of FIG. 3 is shown attached to immersion tube 144 in the same manner as burner 10 is attached to metallic furnace casing 15. The basic difference between the two burners is that the air supplied to combustion chamber 24 through radial apertures 41 in burner 10 is supplied in the burner 110 of FIG. 3 in part to the combustion chamber 124 through apertures 141 and in part directly to immersion tube 144 through spin vanes 145 circumposing the combustion chamber adjacent its outlet end.

Spin vanes 145 are attached to flange member 146 over which immersion tube 144 is slipped for mounting to burner body 112. Spin vanes 145 thus occupy the annular space between flange member 146 and combustion chamber 124. Vanes 145 are attached to flange member 146 at some angle, shown nominally as 45 to the axis of flange member 146 and combustion chamber 124 to cause the air to spin a desired amount. The spacing between vanes is designed to proportion the ratio of aperture 141 area to the effective annular area of the spin vane assembly to thus proportion the ratio of air admitted radially to the combustion chamber and the amount admitted to immersion tube 144 through spin vanes 145.

Spin vanes 145 impart a spiral motion to the air passing through them into immersion tube 144 so that this air spins around the axially flowing fuel and products of combustion stream issuing from combustion chamber 124, and the spinning lair also travels axially down tube 144. Ttbe spiralling air tends to produce a slow mixing long flame which travels the length of the immersion tube. The spiralling air eliminates hot spots and produces a uniform temperatures distribution over the length of the immersion tube by the combined effect of lengthening the flame and providing a spiralling layer of air adjacent the inside wall of the tube through which the heat must pass. The amount of air which passes through the spin vanes is normally regulated at 70-75% of the total air.

FIG. 4 shows another modification of the burner adapted to fire into a commonly used hair-pin type radiant tube 247. Burner 210 is the same as burner 110 of FIG. 3 having a spin vane assembly, except that a burner body L112 or air housing is not placed around the combustion chamber 224. Air is supplied to one of the tangential air inlet pipes 239 under pressure to provide a spinning, slow mixing, stable flame 'at the point of entry of the fuel through pipe 227 to combustion chamber 224. An eductor 248 is coupled to the exhaust end of radiant tube 247 to produce a draft or suction through the tube to causev air at atmospheric pressure to flow into combustion chamber 224 through radial apertures 241 and through the spin vanes into tube 247. The draft also pulls in the fuel through pipe 227 which is in substantially axial alignment to combustion chamber 224. Additional air is drawn into tangential air inlet 238 as the firing rate is increased by increasing the draft produced by eductor 248; this air adds to the spinning air admitted under pressure to air inlet 239. Burner 210 produces a long flame in radiant tube 247, and it operates in essentially the same manner as burner 110 except that the air is drawn into the burner by an eductor at the exhaust end of the tube instead of being supplied under pressure to the burner.

The burner 310 illustrated in FIGS. 5 and 6 may be contrasted to burner 10 of FIGS.'1 and 2 by the fact that it is essentially a long-flame burner. Such a burner, which may be operated on either the excess air principle or as a proporti'oning burner, or on some combination of the two, is well suited for heating relatively high temperature furnace chambers (say over 1200 F.) where there is little danger of quenching a long flame. The burner 310 comprises tunnel block 311 attached to burner body 312 which is shown constructed of a weldment but which may also be constructed, at least partially, of castings in the same manner as the burner of FIGS. 1 and 2. Burner body 312 is attached to metallic casing 315 of furnace wall 314 by means of studs 316 aflixed to casing 315 and extending through holes 317 in burner body 312. Nuts 31'8 threaded on studs 316 securely hold the burner body to the casing, and the studs are so positioned that the body is properly aligned with respect to cylindrical tunnel chamber 313.

Burner body 312 has backplate 319 attached thereto by means of continuous weld 351. Disposed within burner body 312 are wall means forming a substantially cylindrical combustion chamber 324 axially aligned with tunnel chamber 313 and welded to backp'late 319. A fuel inlet pipe 327 is inserted through an aperture 35-2 in backplate 319 and 'is afiixed in place by continuous weld 356 in such a manner that the outlet end 32711 of fuel inlet 327 is for reasons which are to be indicated later, located at a distance somewhat downstream, with respect to the direction of fuel flow from backplate 319. Pipe 327 is in substantially axial alignment with combustion cbamber 324.

A portion of the combustion air, preferably from 4- 10% of the amount required for stoichiometric combustion of the fuel stream added through inlet 327, is admitted tangentially to combustion chamber 324 from annular air chamber 342, defined by burner body 312 and combustion chamber 324, by means of air inlet conduits 338 and 339. Conduits 338 and 339 are defined 'minates flush with the on three sides by channel-shaped members 354 and 355, which are adapted to be inserted in milled slots (not shown) in the end of combustion chamber 324 and on the fourth side by a portion of backplate 319. Air inlet conduits 338 and 339 must be located very closely adjacent backplate 319, a feature which will inhere in the illustrated mode of construction, and the axes thereof must be perpendicular to the axis of combustion chamber 324.

Burner 310 differs primarily from burner 10 in that only a very small portion of the required combustion air is added to combustion chamber 324 whereas all of the required combustion air is added to combustion chamber 24 of FIG. 1. Thus burner 310 of FIGS. 5 and 6 incorporates the spin stabilizing feature of burner 10 of FIGS. 1 and 2 but it does not utilize the radial addition of the main or secondary air stream. In burner 310 the main air stream is combined with the unburned fuel stream passing from combustion chamber 324 in a region downstream thereof, i.e. in tunnel 316. This is accomplished by mounting body 312 to furnace casing 315 in such a fashion that annular chamber 342 is in fluid communication with tunnel block 313. Thus, air will pass from chamber 342 into tunnel block 313 circumposing the fuel flowing from combustion chamber 324. To provide for rapid mixing (and hence rapid combustion) in tunnel block 313 of the air stream from chamber 342 and the fuel stream from tube 327 an orifice plate 356 is inserted in the annular space defined by the inside diameter of burner body '312 and the outside diameter of combustion chamber 324. The high degree turbulence imparted to the air stream by virtue of sudden re-ex-pansion occurring downstream of orifice plate 356 greatly increases the rate of intermixing "of the fuel and air streams in tunnel 313 over the rate of intermixing which would occur through natural turbulence.

Although it would be possible to construct orifice plate 356 as a simple annular plate attached either to the inner surface of burner body 312 or, preferably, to the outer surface of combustion chamber 324 (Le. with an inner diameter slightly greater than the outer diameter of chamber 324 and an outer diameter substantially less than the inner diameter of burner body 312) it is preferred to use a more complex construction for reasons to be explained later. The more complex construction comprises an annular plate which would substantially block the annular space between burner body 312 and combustion tube but for a plurality of radial slots 357 formed around the outer periphery thereof. Such a construction, as has been found by experimentation, reduces to a minimum the suction imposed on the outlet end of chamber 324 by the turbulent motion of the air stream downstream thereof.

An excessive suction in this region will undesirably tend to draw the flame front from its stabilized position within combustion chamber 324.

The fact that there are conditions downstream of combustion chamber 324 which may create a slight suction or negative pressure the-rein, whereas combustion chamber 24 of burner '10 of FIGS. 1 and 2 normally operates at positive pressure by virtue of the staged radial admission of secondary air through ports 41, leads to another importantstructural distinction between burner '10 and burner 310'. In burner 10 the end of fuel pipe 27 terinner surface of backplate 19 whereas the fuel inlet pipe 327 of burner 3'10 terminates at a point somewhat downstream, with respect to the direction of fuel flow, from backplate 319. The extended fuel inlet construction of burner 310 functions in the following manner to overcome the tendency of flame front to drift away from the backplate under the influence of downstream suction conditions: the stream of rapidly spinning air, which is added to combustion chamber 324 through tangential air inlet conduits 338 and 339, forms a vortex with a. concomitant vortex suction effect immediately downstream of the outlet end 327a of fuel inlet pipe 327; the vortex suction in the region ofoutlet end 327a tends to divert an outer annular layer of the fuel stream passing therefrom and causes the diverted portion to flow, in reverse in an annular layer immediately outside of the extended portion of inlet 327, toward backplate 319; the diverted portion of the fuel stream mixes with the spinning air stream to form a spinning cornbustible mixture adjacent backplate 319; the spinning combustible mixture can be readily ignited by holding a torch or match to small port '334 because of the fact that flame will pass therethrough since combustion chamber 324 is under suction; the spinning combustible mixt-ure, when once ignited, will continue to burn forming a spinning highly stable flame front which will serve to stabilize the main combustible reaction between the uncombusted fuel stream passing from combustion chamber 324 and the main air stream passing through slotted orifice plate 356. Thus, burner 310 of FIGS. and 6, like burner of FIGS. 1 and 2 and burners 110 and 210 of FIGS. 3 and 4, is self-stabilizing.

FIGS. 7-9 illustrate a radiant tube burner 410 embodying the self-stabilizing feature of burners .10, 110, 210 and 31a. Radiant tube burner 410 differs from the radiant tube burner 210 in function, by virtue of the fact that it is specifically suited to fire a single-pass radiant tube whereas burner 210 is primarily suited to fire a radiant tube with a plurality of passes such as the illustrated hairpin tube. Radiant tube burner 410 differs further from burner 210 in that it, like the immersion tube burner 110, is specifically adapted to be fired under pressure.

The combustion of a fuel stream within a narrow elongate radiant heating tube may be achieved either according to the principle of pre-mix combustion or according to the principle of diffusion combustion according to some combination of the principles of pre-mix combustion and diffusion combustion. Pre-mix combustion, by definition, is the combustion of a stream of homogeneously mixed fuel and air, normally mixed in approximately stoichiometric proportions. Diffusion combustion, on the other hand, is the combustion of the combustible mixture of fuel and air which occurs by diffusion at the interface of contiguously flowing streams of fuel and air. Pre-mix combustion is normally characterized by a flame which is relatively short and highly stable whereas diffusion combustion is characterized by a flame which is quite long and relatively unstable. Some prior artisans have found it expeditious to offset these relative merits and demerits of pre-mix and diffusion combustion by combining these combustion principles into a combustion technique known as partial pre-mix partial diffusion. An example of this technique is described in US. Patent 2,873,798 to Knight which discloses a radiant tube burner of the diffusion type piloted by a cluster of pre-mix pilot burners.

Simple diffusion combustion has long been widely recognized as an ideal combustion technique for multiplepass radiant tube elements which are commonly constructed in the shape of a U or a W. In such radiant tubes there is normally available a relatively large combustion volume when compared to the combustion volume available in single-pass radiant tubes, the additional volume being obtained in the additional passes of the multiple pas-s tube which greatly increase the length to diameter (L/D) ratio of the tube. Because of the greater L/D ratio of a multiple-pass radiant tube, the extremely long flame characteristic of a diffusion burner is not only tolerable but is even desirable in that it promotes more uniform heat release from all portions of the radiant tube surface. Consequently, pre-mix combustion is commonly employed only to a limited extent in prior art multiple pass radiant tube firing systems and usually only as an expedient to overcome the poor flame stability characteristic of prior art diffusion burners.

Certain industrial furnace designs are, however, because of space limitations or other factors, not readily suited to being heated by a bank of multiple pass radiant tubes.

Such furnaces can be conveniently heated only by a plurality of single-pass radiant tubes. Examples of such a furnace are described in US. Patents 2,822,798 and 2,849,- 220. The single pass radiant tubes used to heat such furnaces are characterized by a relatively small length to diameter ratio when contrasted with the length to diameter ratios of multiple pass radiant tubes. Because of the small length to diameter ratio of single pass radiant tubes, it has heretofore not been practicable to fire such radiant tubes with conventional diffusion burners due to the excessive flame length characteristic of prior art diffusion burners. Accordingly it has been the practice of prior artisans to fire such single pass radiant tubes with partial diffusion, partial pre-mix burners using a much higher degree of pre-mix combustion than was needed for mere stabilization of the diffusion combustion flame.

The use of pre-mix combustion in any radiant tube firing system creates numerous problems many of which become especially severe in single pass radiant tube firing systems which, heretofore, have required a very high degree of pre-rnix combustion to obtain proper flame length control. First, the use of any degree of preamix combustion greatly complicates the construction of the burner and its piping system, factors which add considerably to the cost of the firing system. Secondly, the use of pre-mix combustion, with its concomitant rapid rate of heat release, tends to overheat the entrance portion of the radiant tube imediately downstream of the burner. This problem can be quite severe by virtue of the fact that this portion of the radiant tube is normally surrounded by the thick refractory material with which the furnace is lined, such refractory tending to retard heat transfer rfom the shielded portion of the radiant tube.

Burner 410 of FIGS. 7-9 is shown attached to singlepass radiant heating tube 447 which passes through, and is adapted to deliver heat to, furnace chamber 461 which is defined by wall means 414 of refractory construction backed up by metallic casing plate 415. Radiant tube 447 is disposed vertically in chamber 461 and is supported on annular retaining plate 462 which is attached to casing plate 415 by means of studs 463 and nuts 464. Burner 410 is mounted in coaxial fluid communication with radiant tube 447 by attaching it to annular retaining plate 462, as by means of studs 465 and nuts 466. Burner 410 comprises burner body 412 and a backplate 467 affixed thereto by means of bolts 468 and nuts 469 with a gasket 471 inserted between burner body 412 and backplate 467 to seal the joint therebetween. Extending through a hole in backplate 467 is a combustion chamber 424 formed by cylindrical wall 472 and backplate 419, it being noted that combustion chamber 424 ought to be substantially coaxial with burner body 41 2. Extending through a hole in backplate 419 and terminating at a point somewhat downstream (with respect to the direction of fuel flow therethrough) is a fuel inlet pipe 427 which is in substantially axial alignment with combustion chamber 424.

A stream of air comprising 4-l0% of the amount of air required for stoichiometric combustion of thestream of fuel flowing thro'ugh fuel inlet pipe 427 is added tangentially to combustion chamber 424 by means of air inlet pipe 438. Air inlet pipe 438 is located upstream of the outlet end 427a of fuel inlet tube 427 and is disposed with its axis perpendicular to the axis of combustion chamber 424 so that the stream of air which passes into combustion chamber 424 has substantially no component of motion parallel to the axis of combustion chamber 424. A spinning stable ring of flame can be obtained adjacent backplate 419 of burner 410 in the same manner that a stable ring of flame is obtained adjacent backplate 319 of burner 310 except that, in the case of burner 410', the

flame front can sometimes be caused to shift from a region adjacent backplate 419 to a region adjacent the outlet end of combustion tube 424 due to suction on combustion tube caused by tangential admission of main air to burner body 412. The shifting of the flame front in this fashion has not been found to be particularly undesirable and the flame front will normally recede to a region adjacent the backplate when the suction on combustion tube 424 is reduced as the rate of admission of main air to burner body 412 is reduced on normal turn-down. The only noteworthy aspect of the shifting flame front phenomenon is that, in instances where flame monitoring is desired, it is necessary to monitor at a point downstream of combustion chamber 424. This may be acornplished by inserting a conventional flame detecting rod (not shown) through fragmentary pipe coupling 473 attached to burner body 412 coaxially with aperture 474 and being disposed with its axis adapted to intersect the axis of combustion chamber 424 at a point downstream thereof.

Ignition of the flame stabilizing ring in combustion chamber may be accomplished by means of a spark plug (not shown) inserted through fragmentary pipe coupling 475 attached to combustion chamber 424 coaxially with aperture 476 and being disposed with its axis adapted to intercept the axis of fuel inlet pipe 427 at a point downstream thereof.

The main stream of combustion air for combusting the unburned portion of the fuel stream emerging from combustion chamber 424 is added tangentially by means of air inlet connection 443 to the annulus 477 formed between the outside diameter of combustion chamber 424 and the inside diameter of burner body 412. It is especially important in a burner adapted for firing a single pass radiant tube that air inlet connection be disposed with its axis substantially perpendicular to the axis of combustion chamber 424. Such a construction imparts a very rapid tight spinning motion to the stream of main combustion air as contrasted to the lazier spiraling motion imparted to the stream of main air by turning vanes 145 of burner 110. Due to the relatively high vortex suction effect with an associated high degree of turbulence at the axis of a tightly spinning air stream the fuel stream flowing along the axis will tend to mix very rapidly with the spinning air stream. The mixing occurring in this fashion will proceed in a pattern starting from the axis outwardly to the inner periphery of radiant tube 447. Thus, the extreme outer layer of the spinning air stream will retain its autonomy for a sufliciently long period to provide for effective high convective cooling of firing end portion 447a of radiant tube 447 which is shielded by a portion of refractory wall 414 and which, because of the limited ability of refractory wall 414 to absorb heat therefrom, is a portion particularly susceptible of being overheated.

In constructing bumer 410 it is important to pay careful attention to the size of annulus 477 so that it is neither too large or too small. I have found that the optimum annulus size is one that will impart a pressure drop of 4-5 in W.C. to the stream of spinning air flowing therethrough when air is being added at the maximum desired rate. A smaller annulus will impart a higher pressure drop to the air stream and this will undesirably tend to dampen the spinning motion of the air stream. A larger annulus will impart a lower pressure drop to the air stream and this will undesirably reduce the rate of mixing of the spinning air stream and the axially flowing fuel stream.

The best modes known to me to carry out this invention have been described in terms sufliciently full, clear, concise, and exact as to enable any person skilled in the art to practice the invention. However, it is to be understood that various modifications of the above will be readily apparent to a skilled artisan without departing from the scope of the invention which is defined only by the appended claims.

I claim:

1. Self-stabilizing burner apparatus for heating a tubular element comprising, in combination: wall means including a backplate forming a substantially cylindrical combustion chamber having an inlet end and an exit end, said chamber being in fluid communication with said tubular element; a burner body circumposing the exit end of said combustion chamber and forming therewith an annulus in fluid communication with said tubular element; fuel pipe means for delivering a stream of fuel substantially axially to said combustion chamber, said fuel pipe means having an outlet end disposed within said combustion chamber somewhat downstream from the backplate thereof; first air inlet pipe means for delivering a stream of air to said combustion chamber, said first air inlet pipe means being disposed tangential-1y to said combustion chamber and perpendicularly to the axis thereof and being disposed intermediate said backplate and the outlet end of said fuel pipe means; and second air inlet pipe means for delivering a stream of air to said annulus, said second air inlet pipe means being disposed substantially perpendicularly to the axis of said combustion chamber.

2. Burner apparatus according to claim 1 wherein said annulus is sized to impart a pressure drop of from 45 inches water column to the stream of air admitted through said second air inlet pipe means when said stream is flowing at the maximum desired rate.

References Cited by the Examiner UNITED STATES PATENTS 2,673,726 3/ 1954 Oldenkamp.

2,787,318 4/1957 Wolfersperger 1581.5 2,806,517 9/1957 Te Nuyl 1581.5 X 2,952,307 9/1960 Schramm et al 1587 3,030,773 4/1962 Johnson 15828.1 X

FOREIGN PATENTS 1,033,539 4/1953 France.

255,080 9/ 1927 Great Britain.

FREDERICK L. MATTESON, JR., Primary Examiner.

ROBERT A. DUA, Assistant Examiner. 

1. SELF-STABILIZING BURNER APPARATUS FOR HEATING A TUBULAR ELEMENT COMPRISING, IN COMBINATION: WALL MEANS INCLUDING A BACKPLATE FORMING A SUBSTANTIALLY CYLINDRICAL COMBUSTION CHAMBER HAVING AN INLET END AND AN EXIT END, SAID CHAMBER BEING IN FLUID COMMUNICATION WITH SAID TUBULAR ELEMENT; A BURNER BODY CIRCUMPOSING THE EXIT END, OF SAID COMBUSTION CHAMBER, AND FORMING THEREWITH AN ANNULUS IN FLUID COMMUNICATION WITH SAID TUBULAR ELEMENT; FUEL PIPE MEANS FOR DELIVERING A STREAM OF FUEL SUBSTANTIALLY AXIALLY TO SAID COMBUSTION CHAMBER, SAID FUEL PIPE MEANS HAVING AN OUTLET END END DISPOSED WITHIN SAID COMBUSTIONGHAMBER SOMEWHAT DOWNSTREAM FROM THE BACKPALTE; FIRST AIR INLET PIPE MEANS FOR DELIVERING A STREAM OF AIR TO SAID COMBUSTION CHAMBER, SAID FIRST AIR INLET PIPE MEANS BEING DISPOSED TANGENTIALLY TO SAID COMBUSTION CHAMBER AND PERPENDICULARLY TO THE AXIS THEREOF AND BEING DISPOSED INTERMEDIATE SAID BACKPLATE AND THE OUTLET END OF SAID FUEL PIPE MEANS; AND SECOND AIR INLET PIPE MEANS FOR DELIVERING A STREAM OF AIR TO SAID ANNULUS, SAID SECOND AIR INLET PIPE MEANS BEING DISPOSED SUBSTANTIALLY PERPENDICULARLY TO THE AXIS OF SAID COMBUSTION CHAMBER. 